(1) Field of the Invention
The present invention relates to an apparatus and a method of drilling metal with short pulses of energy so as to minimize the formation of channels or filaments in the drilled hole.
(2) Description of Related Art
For ultrashort-pulse laser drilling of a hole, a small channel (20-80 xcexcm in diameter) can be formed near the peak of the incident laser fluence at the bottom of features being drilled or cut. In stainless steel, this channel is formed for fluences above approximately 5 J/cm{circumflex over ( )}2. Within this channel, the material removal rate can be much higher than that for the region surrounding the channel. Removal of the material gives rise to a filament extending below the surface of the hole. As used herein, xe2x80x9cultrashort-pulse laser drillingxe2x80x9d refers to drilling with an ultrashort pulse laser producing a laser pulse with a duration less than 20 picoseconds.
With reference to FIG. 1, there is illustrated an idealized cross section of an ultrashort-pulse laser drilled hole 10, during the process of drilling, exhibiting the above noted characteristics. The distance from the surface 17 of the drilling material 13 to the rough bottom 15 of the hole 10 is designated xe2x80x9caxe2x80x9d. The distance from the highest to lowest points of the bottom 15 of the rough hole 10 is designated xe2x80x9cbxe2x80x9d. The distance from the outer surface 17 of the drilling material 13 to the bottom of the filament 19 is designated by xe2x80x9ccxe2x80x9d.
It is important to control the creation and propagation of this small filament because its occurrence increases the likelihood of backwall damage in certain drilling applications. When the filament 19 breaks through the back surface 21 of the drilling material 13 quickly, laser light impinges on any material behind the work piece for significant amounts of time. Elimination of this type of filament growth will enable termination of the drilling process shortly after the hole bottom 15 reaches the back side of the work piece, thus forming the hole 10.
What is therefore needed is a method for laser drilling holes and slots in metal which substantially reduces or eliminates the formation of filaments.
Accordingly, it is an object of the present invention to provide a method of drilling metal with short pulses of laser energy so as to minimize the formation of channels or filaments in the drilled hole.
In accordance with the present invention, a method for removing material via a laser so as to reduce the formation of channels comprises the steps of emitting a laser pulse comprising a pulse energy, a pulse duration, and a fluence towards a surface of a drilling material the fluence of a value sufficient to avoid the formation of a channel in the drilling material at the surface to form a hole comprising a side wall and a bottom, shaping a spatial profile of the laser pulse such that the fluence is substantially uniform across the spatial profile; and emitting at least one subsequent laser pulse having a pulse energy, a pulse duration, and a fluence sufficient to avoid the formation of a channel at the bottom of the hole.